Usually used if there is a chance that you might need to render assistance to someone in a hurry, and your reason for being away is flexible and not so important that you can't leave it or even cancel it altogether.
"They should still be in the building - they said they were awailable. Probably just chatting with a friend."
"I'm getting some lunch. Seeing as you have that important call, consider me awailable if you need assistance, and I'll come help you out. I'll be down the hall."